Poly-Axial Implant Fixation System

ABSTRACT

A fixation system comprises a rod, a fastener including a head and an elongated shank, a yoke and an insert. The yoke includes opposite arms defining a slot to receive the rod, an opening to receive the shank, a surface for supporting the fastener head, and a cross bore through each of the arms. The insert has a base configured for slidable insertion between the yoke arms, and defining a rod supporting surface and an opening sized to receive the fastener shank. The insert further includes a pair of resiliently deflectable fingers extending from the base and including a tab projecting therefrom configured to be received within a cross bore. Each tab defines a beveled face configured to bear against the yoke as the insert is advanced into the yoke slot and to deflect the fingers inward until the tabs are aligned with the cross bores.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to co-pending provisional applicationSer. No. 61/261,597, filed on Nov. 16, 2009, and incorporates byreference herein the entire specification of this provisionalapplication.

BACKGROUND

The present invention relates to implant fixation systems, andparticularly to systems for engaging an elongated member, such as aspinal rod, to a bone. The invention further pertains to a systemutilizing a poly- or multi-axial bone fastener.

In many orthopaedic procedures, an implant is fixed to a bone tostabilize the bone. One example is for stabilization of a spinalsegment. In one type of spinal fixation system, an elongatedstabilization member, such as a spinal rod, is engaged to adjacentvertebrae by a bone engaging member. The bone engaging member istypically a hook configured to engage certain portions of the vertebralanatomy, or a bone screw adapted to be threaded into vertebral bone.

In orthopaedic stabilization systems, and most particularly in spinalsystems, there is a need to accommodate various angular orientationsbetween the elongated member and the spinal segment. This necessarilyrequires an ability to achieve variable angles between the bone engagingmember and the elongated stabilization member. In the simplest case, thevariable angle is in a single plane. In more complex cases, multi- orpoly-axial angular orientations are required. One form of spinalstabilization system utilizes a bone screw having a generallyspherically-shaped head. The screw head is seated within a mating cavityin a yoke. The yoke includes opposite arms forming a U-shaped slot forreceipt of a spinal rod above the screw head.

Various mechanisms have been developed to lock the screw head and therod within the yoke. In one such system, an insert is disposed betweenthe bone screw head and the spinal rod. A set screw threaded into thearms of the yoke presses the rod into the insert, which presses thescrew head into the yoke cavity. When fully tightened, the set screweffectively locks all of the components of the stabilization assemblytogether. The strength or tightness of this fixation requires that allof the components be properly oriented, otherwise no amount oftightening of the set screw will fully lock the assembly together. Sincethe components of the fixation system are typically assembled in situ,the surgeon's ability to visualize the instrumentation is limited. Thereis therefore a need for an implant fixation system that can provideassurances to the surgeon that the components are properly situated sothat the components can be effectively locked together when theinstrumentation is complete.

SUMMARY

In one aspect, a fixation system is provided comprising an elongatedrod, a fastener including a head and an elongated shank having a boneengaging portion, a yoke and an insert. The yoke includes opposite armsdefining a slot therebetween sized for receiving the rod therein, a baseportion defining an opening in communication with the slot sized toreceive the shank of the fastener therethrough and a surface adjacentthe opening for supporting the head of the fastener. In one feature, theyoke further defines a cross bore through each of the arms.

The insert includes a base configured for slidable insertion between thearms of the yoke, the base defining a rod supporting surface configuredto support the rod thereon and an opening sized to receive the shank ofthe fastener therethrough. In a further feature, the insert includes apair of resiliently deflectable fingers extending from the base todefine a slot configured to receive the rod therebetween. Each of theresiliently deflectable fingers include a tab projecting outwardlytherefrom and configured to be received within a cross bore in acorresponding one of the arms of the yoke. Each tab defines a beveledface configured to bear against the opposite arms of the yoke as theinsert is advanced into the slot of the yoke to deflect the resilientlydeflectable fingers toward each other until the insert is advanced farenough into the yoke for the tabs to align with the cross bores in thearms of the yoke. Once the tabs are aligned with the cross bores, theresiliently deflectable arms deflect outward to their naturalconfiguration so that the tabs engage the cross bores.

In a further aspect, each of the tabs may include a flat upper surfaceconfigured to prevent removal of the tab from within a correspondingcross bore by translation away from the base portion of the yoke.

The resiliently deflectable fingers and tabs provide a tactile feel andan audible indication when the insert is fully disposed within the yoke.The fingers are further configured to engage the rod within the insertand yoke to form a stable fixation assembly.

DESCRIPTION OF THE FIGURES

FIGS. 1 a-1 d are front, rear, side and top views of a poly-axialfixation system according to one embodiment.

FIG. 2 is a cut-away view of the fixation system shown in FIG. 1 a.

FIG. 3 is an enlarged view of the components of the fixation systemshown in FIG. 2.

FIG. 4 is a side view of the bone engaging fastener component of thefixation system shown in the prior figures.

FIG. 5 is a perspective view of the yoke component of the fixationsystem shown in the prior figures.

FIG. 6 is a side cross-sectional view of the yoke component shown inFIG. 4.

FIG. 7 is a perspective view of the insert component of the fixationsystem shown in the prior figures.

FIG. 8 is a side view of the insert component shown in FIG. 7.

FIG. 9 is an end view of the insert component shown in FIG. 7.

FIG. 10 is a side cross-sectional view of the insert component shown inFIG. 7.

FIG. 11 is an exploded view of the yoke, fastener and insert componentsshown in the prior figures.

FIG. 12 is a side cross-sectional view of the yoke, fastener and insertcomponents in their assembled configuration.

FIGS. 13 and 14 are perspective views of alternative versions of theinsert component for a fixation system as disclosed herein.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

An implant fixation assembly 10 shown in FIGS. 1 a-d, 2 and 3 is adaptedfor use in spinal fixation and stabilization. The assembly includes abone engaging fastener 12, in the form of a bone screw and a yoke 14that couples the bone screw 12 to an elongated stabilization member inthe form of a spinal rod R. A set screw 16 is provided that clamps thecomponents together with the bone screw 12 in any one of a plurality ofspherical angles relative to the yoke 14. An insert 18 is disposedbetween the rod R and the bone screw.

As shown in FIG. 4, the bone engaging fastener 12 includes a generallyspherically shaped head 20 that blends into a smooth shank 22. The shankof the fastener is provided with bone engaging threads 24 configured forengagement within vertebral bone. The head 20 may be provided with afixation feature 26 to enhance the fixation between the fastener and theyoke. For example, the fixation feature 26 may include knurling, aroughened surface, or a pattern of grooves that can engage an innersurface of the yoke 14 when the assembly is tightened. The head 20 ofthe fastener 12 may be provided with an internal feature, such as aninternal hex or Torx™ configuration, for mating with a tool to drive thefastener into bone.

The yoke component 14 is shown in detail in FIGS. 5-6. The yoke 14includes a base portion 30 that defines a cavity 32 for receiving andsupporting the head 20 of the bone fastener and a bore 34 through whichthe shank 22 of the fastener passes. The cavity 32 and bore 34 may beconfigured to permit the bone fastener to move through a range ofspherical angles relative to the yoke 14. However, it is contemplatedthat the yoke and bone screw may be configured for limited movementrelative to each other, such as in a single plane, or configured for norelative movement at all.

The yoke includes a pair of opposite arms 36 that define a central bore38 in communication with the cavity 32. The arms further define aU-shaped slot 40 configured to receive the spinal rod R therein, asshown in FIGS. 1 a-d. The upper end of the arms 36 may include internalthreads 42 configured for threaded engagement with a set screw 16 thatis used to clamp the rod R within the yoke. It is, however, contemplatedthat other structures may be used to fix the rod within the yoke in lieuof the set screw. The yoke further defines a cross bore 44 passingthrough each arm 36. The cross bore cooperates with the insert 18 asdescribed herein to provide the surgeon with a sensible indicator thatthe components are properly oriented when assembled in situ. The crossbore 44 may be further configured for engagement by an insertion toolthat grasps the yoke to facilitate introduction of the yoke and/or bonescrew.

The insert 18 as shown in FIGS. 7-10 includes a base 50 that defines agenerally spherical cavity 52 at the underside of the base. The cavity52 is configured to receive the head 20 of the bone fastener so that thefastener head is contained within an upper and lower cavity formed bycooperation between the insert 18 and yoke 14. The spherical features ofthe yoke, insert and bone screw accommodate multi-axial movement of thebone screw relative to the yoke. As indicated above, the interfacebetween these components may be configured to limit relative movementaccordingly.

The outer diameter of the base 50 of the insert is sized to pass snuglythrough the central bore 38 in the yoke, as shown in FIG. 3. In theillustrated embodiment, the base and central bore are generallycylindrical. However, the components may taper in a conical fashion orassume other complementary configurations that permit sliding movementof the insert 18 along the bore 38. The base defines an opening 51 sizedto receive the shank 22 of the bone screw 12 therethrough.

The insert 18 includes opposite side walls 54 that together with thebase define a channel 56 and a slot 58, as best seen in FIGS. 8 and 10.The channel 56 may be generally cylindrical to define a rod supportingsurface within which the rod is seated. The slot 58 may define agenerally curved interior surface 60 to bear against the rod R when therod is seated within the channel 56 and the insert 18 is disposed withinthe yoke 14. The interface between the rod R and the curved surface 60may incorporate features to enhance engagement of the rod within theinsert, such as knurling, roughening or other fixation patterns oneither the rod, the curved surface or both.

The insert further includes resiliently deflectable fingers 62 situatedbetween or flanked by the side walls 54. The fingers are attached to thebase 50 so that the fingers may deflect relative to the base and to theside walls 54. In one embodiment, the base defines an undercut 71 at theconnection of the fingers 62 to the base 50. The undercut 71 acts as aresilient hinge to permit flexing of the fingers relative to the base50. Alternatively, the resiliently deflectable fingers 62 may beconfigured to bend along their length.

The resiliently deflectable fingers include a generally curved innersurface 63 that may be at least initially contiguous with the interiorsurface 60 of the side walls. The inner surface 63 is configured toengage or substantially conform to the sides of the spinal rod R when itis disposed within the insert and seated on the rod supporting surfaceof the base 50. The rod is thus engaged at three locations—at the bottomof the rod by the curved surface 56 and at its sides by the surface 63of the two resiliently deflectable fingers 62. The resilientlydeflectable fingers 62 include outwardly projecting tabs 66 that mayincorporate a beveled lower face 68 as shown in FIG. 10. The tabs 66extend to an outer diameter that is greater than the outer diameter ofthe base 50 and greater than the inner diameter of the central bore 38of the yoke 18.

The tabs 66 may be provided with inwardly projecting portions 71 thatproject slightly inward across the channel formed by the side walls 54.The portions 71 are spaced apart a distance that is slightly less thanthe diameter of the spinal rod R. These portions 71 provides someresistance to the insertion of a spinal rod between the resilientlydeflectable fingers of the insert. Thus, as the rod is introduced intothe insert, the fingers 62 deflect outward slightly as the rod passesbetween the inwardly projecting portions 71. Once the rod is seatedagainst the curved surface 56, the fingers return to the undeflectedposition shown in FIG. 10 with the portions 71 slightly encircling therod. This configuration helps retain the rod within the insert 18 as itis introduced into the yoke 14.

The components are assembled as shown in FIGS. 2, 3, 11 and 12. The bonefastener 12 is passed through the bore 34 in the yoke 14 and is driveninto the bone a predetermined depth. The yoke is thus initially anchoredto the bone, although the fastener head is not yet fixed within thecavity 32, so that the yoke 14 is free to articulate or angulaterelative to the fastener and bone.

The spinal rod R is placed within the channel 56 of the insert 18 andthe insert is then pushed into the central bore 38 of the yoke 14 withthe rod R aligned with the U-shaped slot 40 of the yoke. As the insertpasses into the central bore, the beveled face 68 of the tab 66 on eachresiliently deflectable finger 62 contacts the top of the yoke. As theinsert is pushed farther into the central bore, the beveled face of thetabs forces the resiliently deflectable fingers 62 to deflect inward tofully engage the sides of the rod R.

The insert is pushed deeper into the yoke until the tabs 66 reach thecross bores 44 in the yoke arms 36, at which point the resilience of thefingers 62 causes the tabs to deflect outward into the cross bores, asshown in FIG. 12. This action between the resiliently deflectablefingers and the yoke cross bores provides both a tactile and an audiblesensory signal that the insert and rod are properly seated within theyoke and over the fastener head. With the insert 18 situated as shown inFIG. 12 the bone screw and rod are provisionally coupled together. Thefingers 62 are configured with a flat upper surface 69 (FIG. 10) so thatthe upper surface engages the top of the cross bores 66 to preventremoval of the insert, or more particularly to restrict retrogrademovement of the insert away from the base portion of the yoke

The cross bores 44 have sufficient depth so that the tabs 66 cantranslate downward toward the cavity 32 of the yoke, as seen in FIGS. 3and 12. Thus, when the set screw 16 is threaded into the internalthreads 42 of the yoke, the set screw 16 bears against the rod R seatedwithin the channel 56 of the insert 18. This in turn pushes the insertdeeper into the yoke central bore and thereby presses the cavity 52 ofthe insert into the fastener head 20. The tabs 66 thus translatedownward within the cross bores 44 until the beveled face 68 contactsthe base of the cross bores. As the set screw is threaded further intothe yoke, the beveled face of the tabs tend to push the resilientlydeflectable fingers inward to engage the rod more tightly. Moreover, anopposite reaction causes the base portion 30 of the yoke 14 to be pulledupward toward the insert 18. When the set screw 16 is fully tightened,the head 20 of the fastener 12 is thus tightly clamped between the yokecavity 32 and the insert cavity 52. Moreover, the rod R is tightlyclamped between the set screw 16 and the channel 56 of the yoke.

Alternative embodiments of the insert are shown in FIGS. 13 and 14. Theinsert 18′ of FIG. 13 includes resiliently deflectable fingers 62′projecting from the base 50′. In this embodiment, the insert does notinclude the side walls 54 found in the insert 18 of FIG. 7. Thus, theengagement of the rod is accomplished solely by the resilientlydeflectable fingers 62′.

In the embodiment of FIG. 14, the insert 18″ includes a base 50″ thatextends upward and forms a rod engaging surface 56″ adapted to receivethe spinal rod R. The upper extent of the base 50″ defines resilientlydeflectable tabs 66″ that project outward to engage the cross bores 44as described above. In this embodiment the insert 18″ does not includethe resiliently deflectable fingers 62 of the prior embodiment so theinsert does not engage the sides of the spinal rod. However, theresiliently deflectable tabs 66″provide the same audible and tactilesensory indication when the inert 18″ is properly positioned within theyoke 14.

It is contemplated that the components of the fixation systems describedherein are formed of medical grade materials, such as stainless steeland titanium. It is further contemplated that the components may beformed by machining, forging or casting in a conventional manner.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected.

1. A fixation system comprising: an elongated rod; a fastener includinga head and an elongated shank having a bone engaging portion; a yokeincluding opposite arms defining a slot therebetween sized for receivingsaid rod therein, a base portion defining an opening in communicationwith said slot sized to receive said shank of said fastener therethroughand a surface adjacent said opening for supporting said head of saidfastener, said yoke further defining a cross bore through each of saidarms; an insert having a base configured for slidable insertion betweensaid arms of said yoke, said base defining a rod supporting surfaceconfigured to support the rod thereon and an opening sized to receivesaid shank of said fastener therethrough, said insert further includinga pair of resiliently deflectable fingers extending from said base todefine a slot configured to receive said rod therebetween, each of saidresiliently deflectable fingers including a tab projecting therefrom andconfigured to be received within a cross bore in a corresponding one ofsaid arms of said yoke, each tab defining a beveled face configured tobear against said opposite arms of said yoke as said insert is advancedinto said slot of said yoke, said beveled face configured to deflectsaid resiliently deflectable fingers toward each other until said insertis advanced far enough into said yoke for said tabs to align with saidcross bores in said arms of said yoke.
 2. The fixation system of claim1, wherein each of said tabs includes a flat upper surface configured toprevent removal of said tab from within a corresponding cross bore bytranslation away from said base portion of said yoke.
 3. The fixationsystem of claim 1, wherein said insert includes side walls flanking eachof said resiliently deflectable fingers.
 4. The fixation system of claim1, wherein said tab of each of said resiliently deflectable fingersincludes an inwardly projecting portion projecting into said slot ofsaid insert and space apart a distance less than a diameter of saidelongated rod.
 5. The fixation system of claim 1, wherein each of saidresiliently deflectable fingers defines a curved interior surfaceconfigure to substantially conform to the surface of said elongated rodwhen said rod is seated on said rod supporting surface.
 6. The fixationsystem of claim 1, wherein each of said resiliently deflectable fingersis connected to said base of said insert by a resilient hinge.
 7. Thefixation system of claim 6, wherein said resilient hinge is an undercutbetween said base and each of said resiliently deflectable fingers. 8.The fixation system of claim 1, wherein each of said resilientlydeflectable fingers is configured to bend along the length thereof. 9.The fixation system of claim 1, wherein said base of said insert definesa surface opposite said rod supporting surface configured to engage saidhead of said fastener.
 10. The fixation system of claim 9, wherein: saidhead of said fastener is substantially spherical; and said oppositesurface is substantially spherical.
 11. An assembly for fixing anelongated rod to a fastener including a head and an elongated shankhaving a bone engaging portion, said assembly comprising: a yokeincluding opposite arms defining a slot therebetween sized for receivingthe rod therein, a base portion defining an opening in communicationwith said slot sized to receive the shank of the fastener therethroughand a surface adjacent said opening for supporting the head of thefastener, said yoke further defining a cross bore through each of saidarms; an insert having a base configured for slidable insertion betweensaid arms of said yoke, said base defining a rod supporting surfaceconfigured to support the rod thereon and an opening sized to receivethe shank of the fastener therethrough, said insert further including apair of resiliently deflectable fingers extending from said base todefine a slot configured to receive the rod therebetween, each of saidresiliently deflectable fingers including a tab projecting therefrom andconfigured to be received within a cross bore in a corresponding one ofsaid arms of said yoke, each tab defining a beveled face configured tobear against said opposite arms of said yoke as said insert is advancedinto said slot of said yoke, said beveled face configured to deflectsaid resiliently deflectable fingers toward each other until said insertis advanced far enough into said yoke for said tabs to align with saidcross bores in said arms of said yoke.
 12. The fixation system of claim11, wherein each of said tabs includes a flat upper surface configuredto prevent removal of said tab from within a corresponding cross bore bytranslation away from said base portion of said yoke.
 13. The fixationsystem of claim 11, wherein said insert includes side walls flankingeach of said resiliently deflectable fingers.
 14. The fixation system ofclaim 11, wherein said tab of each of said resiliently deflectablefingers includes an inwardly projecting portion projecting into saidslot of said insert and space apart a distance less than a diameter ofthe elongated rod.
 15. The fixation system of claim 11, wherein each ofsaid resiliently deflectable fingers defines a curved interior surfaceconfigure to substantially conform to the surface of the elongated rodwhen the rod is seated on said rod supporting surface.
 16. The fixationsystem of claim 11, wherein each of said resiliently deflectable fingersis connected to said base of said insert by a resilient hinge.
 17. Thefixation system of claim 16, wherein said resilient hinge is an undercutbetween said base and each of said resiliently deflectable fingers. 18.The fixation system of claim 11, wherein each of said resilientlydeflectable fingers is configured to bend along the length thereof. 19.The fixation system of claim 11, wherein said base of said insertdefines a surface opposite said rod supporting surface configured toengage said head of said fastener.
 20. The fixation system of claim 19,wherein: said head of said fastener is substantially spherical; and saidopposite surface is substantially spherical.